Cells found in solid tumors differ from healthy cells in a number of ways; notably cancerous cells often exhibit a loss of polarity and the resulting tumor tissues are highly disorganized. Tissue organization arises from precise control of the orientation of cell division which is mediated by microtubules and microtubule-associated proteins. The mitotic spindle, which originates at the nuclear position and is composed of microtubules, aligns perpendicular to the cleavage furrow to ensure proper segregation of cytoplasmic and nuclear materials. Asymmetric division results from asymmetric spatial distribution of cellular components prior to division. Despite the importance of mitotic spindle positioning in specifying the division plane, the mechanics and interactions that link cell polarit cues to the spindle position are unclear. The C. elegans zygote is an ideal model organism for studying the mechanics of spindle positioning because genetic and molecular techniques are well developed. In the single-celled embryo, the female and male pronuclei meet near the posterior and are translocate to the middle of the embryo while simultaneously rotating. Thus, the spindle axis aligns with the anterior/posterior polarity axis of the cell, giving rise to anteror and posterior specified fates after the first division. Studies of spindle orientation have assumed that asymmetric localization of force generators induces torque for spindle rotation. However, mathematical modeling of known interactions between polarity proteins and microtubules results in centering of the nucleus but not rotation. Rotation depends on the microtubule plus-end motor protein dynein; however, dynein is located symmetrically throughout the embryo. Moreover, dynein regulators are localized either anteriorly or posteriorly, but with radial symmetry. Two mechanisms that are not mutually exclusive, might contribute to rotation. The first is that some protein affecting the pulling force of dynein on microtubules is radially asymmetric, leading to more force acting on one of the centrosomes. The second mechanism is that the centrosomes do not respond equally to the force from dynein. The central hypothesis of this proposal is that rotation can be explained by radially asymmetric cortical forces acting on the centrosomes coupled with a maturation-dependent difference in the way centrosomes respond to cortical forces. This hypothesis will be tested with three specific aims: 1) Generate mathematical models of the rotation event to test the two mechanisms of rotation. 2) Investigate the maturation-dependent differences between the two centrosomes. And 3) Using a candidate approach, uncover the radially asymmetric regulator(s) of dynein activity. This research is significant because it will lay the foundation for important questions in cancer and stem cell biology. Mathematical and physical modeling has contributed to our understanding of many biological processes. Models not only help to summarize key data, but also offer predictive power to generate novel hypotheses that can be tested in vivo.